Abstract. Absorption of radiation by ice is extremely weak at visible and
near-ultraviolet wavelengths, so small amounts of light-absorbing impurities
in snow can dominate the absorption of solar radiation at these wavelengths,
reducing the albedo relative to that of pure snow, contributing to the
surface energy budget and leading to earlier snowmelt. In this study Arctic
snow is surveyed for its content of light-absorbing impurities, expanding
and updating the 1983–1984 survey of Clarke and Noone. Samples were collected
in Alaska, Canada, Greenland, Svalbard, Norway, Russia, and the Arctic Ocean during 1998 and 2005–2009, on tundra, glaciers, ice caps, sea ice, frozen lakes, and in
boreal forests. Snow was collected mostly in spring, when the entire winter
snowpack is accessible for sampling. Sampling was carried out in summer on
the Greenland Ice Sheet and on the Arctic Ocean, of melting glacier snow and
sea ice as well as cold snow. About 1200 snow samples have been analyzed for
this study.

The snow is melted and filtered; the filters are analyzed in a specially
designed spectrophotometer system to infer the concentration of black carbon
(BC), the fraction of absorption due to non-BC light-absorbing constituents
and the absorption Ångstrom exponent of all particles. This is done
using BC calibration standards having a mass absorption efficiency of
6.0 m2 g−1 at 550 nm and by making an assumption that the absorption
Angstrom exponent for BC is 1.0 and for non-BC light-absorbing aerosol is
5.0. The reduction of snow albedo is primarily due to BC, but other
impurities, principally brown (organic) carbon, are typically responsible
for ~40% of the visible and ultraviolet absorption. The meltwater
from selected snow samples was saved for chemical analysis to identify
sources of the impurities. Median BC amounts in surface snow are as follows
(nanograms of carbon per gram of snow): Greenland 3, Arctic Ocean snow 7,
melting sea ice 8, Arctic Canada 8, subarctic Canada 14, Svalbard 13,
Northern Norway 21, western Arctic Russia 27, northeastern Siberia 34.
Concentrations are more variable in the European Arctic than in Arctic
Canada or the Arctic Ocean, probably because of the proximity to BC sources.
Individual samples of falling snow were collected on Svalbard, documenting
the springtime decline of BC from March through May.

Absorption Ångstrom exponents are 1.5–1.7 in Norway, Svalbard, and
western Russia, 2.1–2.3 elsewhere in the Arctic, and 2.5 in Greenland.
Correspondingly, the estimated contribution to absorption by non-BC
constituents in these regions is ~25%, 40%, and 50%
respectively.

It has been hypothesized that when the snow surface layer melts some of the
BC is left at the top of the snowpack rather than being carried away in
meltwater. This process was observed in a few locations and would cause a
positive feedback on snowmelt.

The BC content of the Arctic atmosphere has declined markedly since 1989,
according to the continuous measurements of near-surface air at Alert
(Canada), Barrow (Alaska), and Ny-Ålesund (Svalbard). Correspondingly,
the new BC concentrations for Arctic snow are somewhat lower than those
reported by Clarke and Noone for 1983–1984, but because of methodological
differences it is not clear that the differences are significant.
Nevertheless, the BC content of Arctic snow appears to be no higher now than
in 1984, so it is doubtful that BC in Arctic snow has contributed to the
rapid decline of Arctic sea ice in recent years.